1. Field of The Invention
This invention relates to a method of relaying mobile-radio signals which ensures the communication link between a mobile unit(s) and a land site when the mobile unit(s) enters and/or is located in a zone in which communication is not normally possible due to weak signal strength, and also relates to a booster used therefor.
2. Description of the Prior Art
Unlike the microwave communications between fixed stations at high elevation, commercial mobile communication services have encountered a problem that because of weak signal strength, communication tends to be blocked in zones such as underground parking lots, tunnels, in the shadows of large buildings, etc.
At the inception of mobile communication services, it was difficult to eliminate the problem due to high costs and unmatured techniques. However, with the increasing number of subscribers the demand for a solution to this problem has increased markedly. In this specification, by definition, the term "zone of weak signal strength" or "weak signal strength zone" describes a zone in which electromagnetic field strength is not strong enough to enable direct mobile-radio communication between the mobile unit and the land site and hence signal boosting is necessary.
One known approach to solving the problem is to provide a relay station at each zone of weak signal strength to amplify all the channels allocated to a coverage area. This approach however, has encountered the difficulty that intermodulation interferences occur when simultaneously amplifying the large number of communication waves such that all the channel signals can not be linearly amplified. Additionally, such a relay station is bulky and complicated with the attendant disadvantage of high manufacturing costs.
Another known approach is to provide the same equipment as a land site in each zone of weak signal strength. However, since the number of mobile units concurrently in use in a single zone is very small, providing such equipment is relatively expensive and thus impractical from an economical viewpoint.